As the optical communication technology moves forward, development of optical components capable of directly processing optical signals becomes increasingly important. Above all, a waveguide-type optical interferometer utilizing interference of light in an optical waveguide structure integrated on a planar substrate is superior in mass production and has advantageous features such as low cost and high reliability. Typically included are an arrayed waveguide grating, a Mach-Zehnder interferometer, and a lattice circuit, for example.
Standard photolithography and etching as well as glass deposition technology such as FHD (flame hydrolysis deposition) are used as a basic method for fabrication of the waveguide-type optical interferometer. A procedure for the fabrication involves, first, depositing on a substrate an undercladding layer and a core layer with a higher refractive index than its surroundings, and then, forming a waveguide pattern on the core layer. The fabrication is accomplished by, further, burying the core layer beneath an overcladding layer. Signal light propagates through the waveguide-type optical interferometer, as being confined within a waveguide formed by the buried core layer.
FIG. 1 is a diagram showing the configuration of an asymmetric Mach-Zehnder interferometer (hereinafter called “asymmetric MZI”) constructed of the waveguide-type optical interferometer. In the asymmetric MZI, an input waveguide formed of a first input waveguide 101 and a second input waveguide 102 is connected to one end of an optical splitter 103. A long arm waveguide 107 and a short arm waveguide 108 of different lengths are connected to the other end of the optical splitter 103. The long arm waveguide 107 and the short arm waveguide 108 are connected to one end of an optical combiner 106. Further, the other end of the optical combiner 106 is connected to an output waveguide formed of a first output waveguide 104 and a second output waveguide 105. The configuration and operation of such an asymmetric MZI are well known and are not described in detail here.
FIGS. 2A and 2B are graphs showing transmission characteristics of the asymmetric MZI. The transmission characteristics refer, for example, to the transmission characteristics across a cross port between the first input waveguide 101 and the second output waveguide 105. FIGS. 2A and 2B each represent a loss spectrum, since the horizontal axis of each graph indicates wavelength as expressed in antilogarithmic form, and the vertical axis thereof indicates loss as expressed in logarithmic form. As shown in FIG. 2B, the loss spectrum is a periodic characteristic. The period of the loss spectrum is inversely proportional to a difference between the optical path lengths of light propagating through the arm waveguides 107 and 108, respectively. The optical path lengths are each expressed by the integral value of the refractive index along the optical path of the propagating light.
Generally, in the waveguide-type optical circuit, a material for forming the waveguide is birefringent. Hence, there are changes in various characteristics of the optical circuit, resulting from birefringence, depending on the state of input polarization. More specifically, in the waveguide-type optical circuit, the substrate, the cladding layer, the core layer, and the like are formed of different materials. Thus, the materials have different coefficients of thermal expansion, which in turn bring about the birefringence. In the above-mentioned procedure for the fabrication of the optical circuit, the materials undergo the process of heat treatment at a high temperature of 1000 degrees or more, and hence, under normal temperature conditions, thermal stresses of very great magnitude appear across parts of the waveguide. A photoelastic effect caused by the thermal stresses leads to the occurrence of the birefringence in the waveguide.
FIG. 2A is the graph showing the transmission characteristics of the asymmetric MZI, provided that the birefringence occurs. Owing to the birefringence, the loss spectrum characteristic varies according to the state of polarization of incoming light, and thus, polarization dependence develops in the loss spectrum characteristic. The reason is that, depending on the state of the input polarization, variations in the refractive index experienced by the propagating light occur and thus cause a slight variation in the period of the loss spectrum. The slight variation in the period appears as a shift in the loss spectrum characteristic along the wavelength (or frequency) axis, provided that the loss spectrum is observed with respect to a given waveband. The amount of shift varies according to the state of the input polarization, and thus the polarization dependence develops in the circuit characteristics. Signal light from a light source for use in an actual optical system is light in various states of polarization combined together, and hence the polarization dependence is an important problem involved in the waveguide-type optical circuit. One of indices indicative of the extent of the polarization dependence is PDf (Polarization Dependence frequency shift). As for the PDf, a difference between a maximum shift toward higher frequencies (or toward shorter wavelengths, corresponding to polarization 1 shown in FIG. 2A) and a maximum shift toward lower frequencies (or toward longer wavelengths, corresponding to polarization 2 shown in FIG. 2A) is called the PDf (Polarization Dependence frequency shift), provided that light in every state of polarization comes in. There is a demand for a reduction in the PDf in the interferometer.
(Conventional Art 1)
There have been proposals of several methods for solving the above-mentioned problem of the polarization dependence. For instance, an amorphous silicon layer or a groove may be formed on or in the surface of the substrate to form a stressing layer and thereby control the birefringence partially or wholly in the waveguide. Control of the birefringence enables a reduction in the polarization dependence throughout the entire optical interferometer (See Patent Document 1). However, such a method has difficulty in stably and reliably suppressing the polarization dependence, since the birefringence varies from one optical circuit to another or from one production lot to another because of manufacturing variability or the like.
(Conventional Art 2)
There has been a proposal of another solving method, which involves placing a polarization mode converter, specifically a half-wave plate, in the interferometer, thereby eliminating the polarization dependence of the interferometer (See Patent Document 2). In this method, the half-wave plate with its optic axis inclined at 45° is placed at the center of the interferometer. The half-wave plate converts horizontal polarization into vertical polarization, and converts vertical polarization into horizontal polarization. This enables elimination of the polarization dependence of the interferometer, for incoming light in a horizontally polarized state or incoming light in a vertically polarized state. As employed herein, the horizontal polarization and the vertical polarization refer to light such that the direction of amplitude of its electric field is horizontal with respect to the plane of the substrate of the optical circuit having the waveguide of rectangular or substantially rectangular cross section, and light such that the direction of amplitude of its electric field is vertical with respect to the plane of the substrate, respectively. This method is an effective means, since the half-wave plate can be used to eliminate the polarization dependence of the interferometer, even if the birefringence in the waveguide varies from one optical circuit to another during the fabrication of the optical circuit.
FIG. 3 is a diagram of configuration of a simple asymmetric MZI. By use of analytical expressions, description will be given below with regard to a polarization dependence elimination operation, which is performed in the interferometer having the half-wave plate interposed therein according to the above-mentioned conventional art 2. In FIG. 3, the optical splitter 103 and the optical combiner 106 are placed at points 3A and 3C, respectively. The optical splitter 103 and the optical combiner 106 are connected by the two arm waveguides 107 and 108 of different lengths. In the asymmetric MZI of such a configuration, the birefringence of the waveguide is designated by B; the refractive index for the horizontal polarization, nTE; the refractive index for the vertical polarization, nTM; a difference in waveguide length between the long arm waveguide 107 and the short arm waveguide 108, δL; and the length of the short arm waveguide 108, 2 L. In the asymmetric MZI shown in FIG. 3 in which the half-wave plate is not placed, an optical path difference δLTE between the two arms is expressed by Equation (1), provided that signal light in a horizontally polarized state comes in.[formula 1]δLTE=(2L+δL)×nTE−2L×nTE=δL×nTE  Equation (1)Meanwhile, an optical path difference δLTM between the two arms is expressed by Equation (2), provided that signal light in a vertically polarized state comes in.[formula 2]δLTM=(2L+δL)×nTM−2L×nTM=δL×nTE  Equation (2)Generally, in the interferometer, the optical path difference determines interference conditions and thus determines interferometer characteristics such as the loss spectrum. As can be seen from Equations (1) and (2), the optical path difference for the horizontal polarization is different from that for the vertical polarization, so that the interference conditions vary according to the state of the input polarization.
FIG. 4 is a diagram showing the configuration of the asymmetric MZI having the half-wave plate interposed at the center. In FIG. 4, the optical splitter 103 and the optical combiner 106 are placed at points 4A and 4C, respectively. The optical splitter 103 and the optical combiner 106 are connected by the two arm waveguides 107 and 108 of different lengths. The horizontal polarization and the vertical polarization change places between before and after a half-wave plate 400 along each of the long arm waveguide 107 and the short arm waveguide 108, by the conversion function of the half-wave plate. Therefore, the optical path difference δLTE between the two arm waveguides is represented as Equation (3), provided that the signal light in the horizontally polarized state comes in.
                    [                  formula          ⁢                                          ⁢          3                ]                                                                                                                δ                ⁢                                                                  ⁢                                  L                  TE                                            =                                                [                                                                                                                                                                        (                                                              L                                +                                                                  δ                                  ⁢                                                                                                                                          ⁢                                                                      L                                    /                                    2                                                                                                                              )                                                        ×                                                          n                              TE                                                                                +                                                                                                                                                                                          (                                                          L                              +                                                              δ                                ⁢                                                                                                                                  ⁢                                                                  L                                  /                                  2                                                                                                                      )                                                    ×                                                      n                            TM                                                                                                                                ]                                -                                  [                                                            L                      ×                                              n                        TE                                                              +                                          L                      ×                                              n                        TM                                                                              ]                                                                                                        =                              δ                ⁢                                                                  ⁢                L                ×                                                      (                                                                  n                        TE                                            +                                              n                        TM                                                              )                                    /                  2                                                                                        Equation        ⁢                                  ⁢                  (          3          )                    Meanwhile, the optical path difference δLTM between the two arm waveguides is represented as Equation (4), provided that the signal light in the vertically polarized state comes in.
                    [                  formula          ⁢                                          ⁢          4                ]                                                                                                                δ                ⁢                                                                  ⁢                                  L                  TM                                            =                                                [                                                                                                                                                                        (                                                              L                                +                                                                  δ                                  ⁢                                                                                                                                          ⁢                                                                      L                                    /                                    2                                                                                                                              )                                                        ×                                                          n                              TM                                                                                +                                                                                                                                                                                          (                                                          L                              +                                                              δ                                ⁢                                                                                                                                  ⁢                                                                  L                                  /                                  2                                                                                                                      )                                                    ×                                                      n                            TE                                                                                                                                ]                                -                                  [                                                            L                      ×                                              n                        TM                                                              +                                          L                      ×                                              n                        TE                                                                              ]                                                                                                        =                              δ                ⁢                                                                  ⁢                L                ×                                                      (                                                                  n                        TE                                            +                                              n                        TM                                                              )                                    /                  2                                                                                        Equation        ⁢                                  ⁢                  (          4          )                    As can be seen from Equations (3) and (4), the optical path difference for the incoming signal light in the horizontally polarized state becomes equal to that for the incoming signal light in the vertically polarized state. In the interferometer, the loss spectrum is determined by the optical path difference. The optical path difference for the horizontal polarization becomes equal to that for the vertical polarization, so that the loss spectrum of the interferometer becomes polarization-independent.
As described above, the half-wave plate 400 is interposed in the interferometer thereby to eliminate the polarization dependence of the interferometer, provided that the horizontally polarized light alone comes in or provided that the vertically polarized light alone comes in. The polarization dependence mentioned above is based on the optical path difference between paths of different lengths inherent in the interferometer. The conventional art achieves the elimination of the polarization dependence developed by the optical path difference and the phenomenon of birefringence.
Patent Document 1: Japanese Patent Laid-Open No. H 07-018964 (1995)
Patent Document 2: Japanese Patent No. 2614365